An optical encoder includes a transmission encoder and a reflection encoder. The transmission encoder includes a light source and a light receiving element that are respectively arranged at one side and the other side of a rotating disc while placing the rotating disc therebetween. The transmission encoder makes light emitted from the light source pass through the rotating disc and makes the light receiving element receive the light. On the other hand, the reflection encoder includes a light source and a light receiving element that are arranged at only one side of a rotating disc. The reflection encoder reflects light emitted from the light source on the rotating disc and makes the light receiving element receive the light. Both of the encoders detect a rotational position and a rotational speed of a rotating shaft, to which the rotating disc is fixed, from an output signal of the light receiving element that receives substantially pulsed light in accordance with the rotation of the rotating disc.
A conventional transmission encoder is disclosed in, for example, Japanese Laid-open Patent Publication No. 2010-96503.
Because all of the light source and the light receiving element in the reflection encoder can be intensively arranged at only one side of the rotating disc, the reflection encoder is suitable for simplification and miniaturization of the configuration of device as compared with the transmission encoder. For this reason, it is the mainstream to employ the reflection encoder in recent years.
However, because a concentric light quantity distribution with the light source at its centre occurs in reflected light in the reflection encoder, it has been concerned that a light receiving area decreases depending on the arrangement of the light receiving element and thus reflected light cannot be effectively utilized.